Cluster formation in fluids with competing interactions

Abstract

Over the past twenty years, an increasing number of researchers have investigated fluids consisting of particles interacting via competing short-range attractions and long-range repulsion (so-called SALR fluids). These are relevant to a wide range of biological, industrial, and nano applications, for example in lysozyme solution. Under a variety of conditions, these fluids self-assemble into a wide range of heterogeneous structures, including a cluster fluid, gel, Wigner glass and several kinds of structured or modulated fluid. This is quite different behaviour from the classic simple fluids which are limited to homogeneous phases such as gas, liquid and crystalline solid, separated by first order phase transition lines. In this thesis, the goal is to further investigate the clustering behaviour of SALR fluids at low density where the cluster fluid phase can form. This area has received relatively less attention in the research literature because the theoretical description of such fluids is very challenging. To be specific, the content of this thesis can be divided into two parts. In the first part, using the Monte Carlo simulation method, the cluster fluid behaviour of symmetric binary SALR mixtures is explored. Pure SALR fluids have been investigated via this route before, but the mixture case has received almost no attention. This thesis reveals some interesting and novel behaviour for the symmetric binary mixture case and the insights could be relevant to asymmetric mixtures. In addition, the non-equilibrium process of cluster fissioning, or reproduction, for the symmetric binary SALR mixture is investigated. In the second part of this thesis, the integral equation method is used to model pure SALR cluster fluids. As already mentioned, the theoretical description of such fluids is very challenging, but the integral equation theory might provide a useful way forward, as it does for simple fluids. The aim here is to access a wide range of cluster fluid states, even those deep within the cluster fluid phase which exhibit large well-defined clusters. Until now, integral equation methods have been able to access only the onset of such clustering behaviour where the system concentration is large. Both simple and complex integral equation closure relations are explored, providing insight into how this route might be used to model the cluster fluid phase more extensively.